1. Field of the Invention
The present invention relates to a chip package structure. More particularly, the present invention relates to a chip package structure having a higher bonding strength between its chip and a substrate.
2. Description of the Related Art
In the present information age, electronic products have become indispensable part of our lives. In general, a chip is at the core of most electronic products. Through the chip, many types of logic or data memory operations can be carried out. With the ever-advancing electronic technologies, more and more functionally powerful and personalized products are produced. Most of these electronic products can have a streamline, compact and miniature appearance due to a reduction of the interconnections linking semiconductor devices from the former 0.25 μm to about 0.15 μm or lower. In fact, the width of interconnections is now closing into the nanometer range.
With many breakthroughs in semiconductor fabrication techniques, many types of high-density semiconductor packages such as the flip-chip packages are produced. In the flip-chip technique, a plurality of bumps is formed on the bonding pads of a chip so that the chip can flip over and electrically connect with a substrate directly through the bumps. Compared with the conventional wire-bonding technique and the tape automated bonding (TAB) technique, the flip chip technique is able to provide a shorter electrical pathway that has a larger average cross-sectional area linking the substrate and the chip. Due to improved performance relative to other types of packages, flip chip packages have become the preferred package type in the semiconductor industry.
After bonding the chip to the substrate through the bumps, a plurality of solder balls may be implanted on the bottom surface of the substrate. Typically, solder balls fabricated from lead-tin alloy are often used because lead has a lower material cost. Furthermore, the process of fabricating lead-tin alloy, its in-process characteristics and reaction with other alloys as well as the preferred flux has been thoroughly investigated. Thus, lead-tin alloy plays an indispensable role in the packaging industry. However, lead is immensely toxic to the human body and hence is one of the major pollutants in our environment.
To reduce the poisoning effect of lead on our environment and human body, a lead-free alloy such as tin-silver-copper alloy has been developed and widely used to fabricate solder balls. Yet, lead-free solder balls need a higher reflow temperature, for example, between 240° C. to 260° C., for joining the lead-free solder balls and the substrate together. At such a high temperature, delamination between the bump and the chip or between the bump and the substrate may occur. Furthermore, delamination between the chip and the underfill material may also occur.
FIG. 1 is a schematic cross-sectional view of a conventional chip package. As shown in FIG. 1, the chip 110 and the substrate 120 are structurally bonded and electrically connected together through the bumps 130. An underfill material is injected into the space between the chip 110 and the substrate 120 to form an underfill layer 140 that encloses the bumps 130. The underfill layer 140 serves as a buffering layer to buffer against the shear stress in the bumps 130 due to a difference in the thermal coefficient of expansion between the chip 110 and the substrate 120. Thereafter, a molding compound 150 is deposited over the substrate 120 to encapsulate and protect the chip 110. Finally, lead-free solder balls are implanted on the bottom surface of the substrate 120 and a reflow process is performed to form a ball grid array (BGA) flip-chip package 100.
In the aforementioned process, the step of reflowing the implanted solder balls or bonding the BGA flip-chip package 100 to an external circuit (not shown) using the surface mount technology (SMT), a high temperature is often required. The heat may cause the molding compound 150 to pull the chip 110 up with a considerable force. The up pulling force on the chip 100 may cause the chip 110 and the bumps 130 or the bumps 130 and the substrate 120 to delaminate. Ultimately, the chip 110 and the underfill material 140 may delaminate as well.